Background:Closed
soilless growing systems have significant advantages: conservation
of scarce water resources, no leaching of nutrients and pesticides
and improved quality of products. Disadvantages of the system
are rapid dispersal of soil-borne pathogens by the recirculating
nutrient solution and accumulation of potential phytotoxic metabolites.
Active disinfection with high-tech equipment suppresses or destroys
the natural microflora. Recent research has demonstrated a natural
suppressiveness by the resident microflora against colonisation
by pathogens. Equally, passive disinfection with slow (sand) filtration
has shown that the resident microflora is not destroyed, while
elimination of some of the most harmful pathogens can be achieved.
A combination of optimising the natural suppression and passive
disinfection will lead to a sustainable technologically-simple
system, which is also inexpensive and robust.

Objectives:The
objective of the project is to develop a sustainable system for
the prevention of root diseases in closed soilless growing systems
in greenhouses, by microbial optimisation. The sustainable system
may replace the soil-grown system, in which the soil fumigant
methyl bromide is used in Southern Europe and the run-to-waste
soilless system in Northern Europe.

Description:To
achieve this sustainable system, the following steps have to be
taken: diseased conditions and in relation to active and passive
disinfection methods; healthy and diseased conditions and, again,
in relation to the disinfection method techniques need to be elucidated
to improve the efficacy of the method. These three steps provide
the basic information on the sustainable concept of disease suppression
in closed systems. The results will be used in the next step:
to enhance the microbial suppression of two root pathogens (Phytophthora
cryptogea, Pythium aphanidermatum) by stimulation and management
of the natural presence of the resident microflora. The final
step will be to inform commercial growers and suppliers of the
results by demonstration of the sustainable system via open days,
a workshop and publication in technical and scientific journals.
The research will be carried out on major crops in protected cultivation
(tomato, cucumber and gerbera). The final outcome will be a sustainable
soilless system which can be adopted in Northern and Southern
Europe, whereby water resources are managed effectively, pesticide
use is minimised and replaced by nonchemical treatments, and the
environment is protected in the long-term.

Current
situation/results:In
the first year, experiments in cucumber, tomato and gerbera have
been executed to optimise and standardise detection techniques
of microflora, present in the nutrient solution, and of metabolites,
released by plant roots or microorganisms. Additionally, slow
filtration experiments have been taken place to optimise the layout
of the filter. Now it can be said that for slow filtration there
is no preferential filter medium. All the media tested (fine filter
sand, "local" sand, glasswool, two types of rockwool
and polyurethane foam) showed no significant differences after
inoculation with the fungus Fusarium oxysporum and the
bacteria Xanthomonas campestris. A similar result was obtained
for the filtration rate (100 and 300 l.m-2.h-1).
Choice of the filter medium should be made on other bases, such
as investment, transport and handling. These experiments were
executed in cucumber and gerbera. In the same crops, samples were
taken to find which groups of microflora were present during cropping
and which detection method (plate count, fatty acid, BIOLOG, DGGE)
is most suitable to detect them. Equally, samples were taken from
a tomato crop. Samples were also taken to detect the lowest possible
concentration. With plate counts, a certain limit exists, but
with PCR and nested-PCR a lower detection limit can be achieved.
The method was optimised for Pythium aphanidermatum and
Phytophthora cryptogea. A dipstick technique was being
optimised to detect the latter pathogens, but variable results
were obtained. Metabolites were tested in low and high concentrations
on tomato seedlings. In general it could be said that low concentrations
improved growth, while high concentrations were toxic.Preparations
were made for testing microbial suppression of Pythium aphanidermatum
and Phytophthora cryptogea while the recirculating nutrient
solution is treated with passive disinfection (slow filtration
at which the natural microflora keeps alive), active disinfection
(UV radiation, the solution is sterilised) and a reference without
any treatment. These tests will be executed again in cucumber,
tomato and gerbera.